1. Field of the Invention
This invention relates to humidity indicator cards and more particularly, to an irreversible humidity indicator card, which is covered on both sides, which utilizes a deliquescent material which does not contain a dye.
2. Description of Prior Art
When shipping or storing many types of materials, particularly electronic components, it is desirable to know whether those components have been exposed to a particular level of humidity which humidity could cause damage to those components. For example, electronic components can be damaged by exposure to low levels of humidity, even where the exposure is only for short periods of time. These same electronic components may also be contaminated and/or damaged by exposure to dust and other particulate matter.
To address the need for the detection of humidity levels within shipping or storage containers, humidity indicators have been developed. There are generally two types of humidity indicators. One of these humidity indicators reversibly changes color upon exposure to particular humidity levels. Such reversible humidity indicators typically utilize cobalt chloride as the humidity indicator material. It changes color when exposed to predetermined levels of humidity and returns to its original color when the humidity level drops below that predetermined level. These reversible humidity indicators are used to indicate the current condition of a desiccant and/or the current humidity level within the storage container.
The second type of humidity indicator is an irreversible humidity indicator. These indicators are designed to detect a predetermined level of humidity and provide a visual indication of whether components stored in the containers where these humidity indicators are used have been exposed to that predetermined level of humidity even for short periods of time and even if the level of humidity drops below that predetermined level when the components are checked at a later time. Large changes in humidity levels sometimes occur where storage containers are used in relatively warm climates where the moisture level in the air rises and falls dramatically depending upon the temperature of the surrounding air. Under these conditions, a reversible humidity indicator might fail to indicate the temporary presence of high humidity within a storage container even though such high humidity may be sufficient to cause damage to the components present in the storage container.
One of the first irreversible humidity indicator devices was disclosed in U.S. Pat. No. 2,214,354, which disclosed the use of a calcium chloride material which was mixed with a water soluble dye and deposited on a porous surface material, such as a sheet of absorbent paper. Upon exposure of the absorbent sheet to a predetermined humidity level, the calcium chloride material liquifies and releases the dye in liquid form. The dye is then carried by capillary action onto the porous surface of the absorbent paper, where it produces a permanent and irreversible dye mark. The aforementioned patent further describes various deliquescent agents which may be employed to show different humidity levels.
To maintain a consistently low humidity level, shipping containers and long term storage containers usually contain desiccant materials. These desiccant materials dehydrate the storage area and are intended to maintain the humidity level within that storage area at a predetermined level. These containers are periodically opened to recharge or replace the desiccant materials placed within the container and/or to check the level of humidity in the storage container. After replacing the desiccant material, the container is again sealed. In order to determine whether the humidity level in these storage containers has ever reached certain critical levels, irreversible humidity indicators are also often placed within the containers with the desiccant materials. These irreversible humidity indicators can be reviewed at the same time that the desiccant material is being checked to determine whether a harmful humidity level has ever been reached in the shipping container.
A series of relative humidity indicators, each utilizing a different deliquescent salt, are disclosed in a series of patents which were issued during the 1940's and 1950's including U.S. Pat. Nos. 2,460,065, 2,460,066, 2,460,067, 2,460,068, 2,460,069, 2,460,070, 2,460,071, 2,460,072, 2,460,073, 2,460,074, 2,526,938, 2,580,737, and 2,627,505. In addition, some humidity indicator cards are capable of showing different levels of humidity on the same card by use of a series of different deliquescent agents that change color at varying humidity levels, as disclosed in U.S. Pat. No. 2,249,867.
Humidity indicator sheets and cards which contain deliquescent salts and dyes have commonly been used to detect the relative humidity level present within storage containers. See for example, U.S. Pat. Nos. 2,249,867, 4,034,609, 4,150,570, and 4,854,160. Button-type humidity indicators or “plug” humidity indicators are also sometimes used with packaging material and are disclosed, for example, by U.S. Pat. Nos. 2,716,338, 3,084,658 and 4,050,307. Another device for monitoring humidity levels, particularly in poured cement, is disclosed in U.S. Pat. No. 3,680,364.
A multiple layer, reversible humidity sensing device containing a reflective layer, which is useful in viewing the changes in color of a humidity indicator card is disclosed by U.S. Pat. No. 4,034,609.
A reversible humidity indicator card contained within transparent, flexible sheet materials with an impermeable front layer is disclosed in U.S. Pat. No. 5,224,373. This humidity indicator card is specifically designed for utilization with electronic components. It is formed as a “window” in a barrier bag.
A delayed action, irreversible humidity indicator card is disclosed in U.S. Pat. No. 4,793,180.
All irreversible humidity indicator cards known hitherto are based on combinations of deliquescent salts and water-soluble dyes. In order to prepare humidity indicator cards that react at various humidity levels, different combinations of deliquescent salts and dyes must be chosen. Only cards that show the same change in color at each chosen humidity level are acceptable to users of these cards. Otherwise, if there are varying color changes, it may be difficult to determine whether a particular humidity level has been reached. W. B. Abel: Chemical Maximum Humidity Indicator Update Report, BDX-613-1989 and U.S. Pat. No. 3,898,172 teach that only certain combinations of salts and dyes are useful for this purpose as the dye is quite soluble in the saturated salt solution that is formed upon deliquescence. In addition, the solubility and color of the dye must be independent of pH changes that may be attributed to the deliquescence of the salt. To ensure a proper shelf-life of the indicator card, the dye also must not react with the salt in any way (e.g. redox reaction, acid-base reaction). It is quite difficult to use either the same dye or different dyes with the same color and different salts over the entire humidity spectrum.
Mixing the individual salts with the dye is an additional required step for the production of the irreversible humidity indicators previously known. If the chosen salt and dye have different particle sizes, inhomogeneous distribution of the dye in the salt may occur and lead to inhomogeneous color and appearance on the indicating spot of the humidity indicator. This problem can be overcome by milling salt and dye together, but this is not possible for all salt—dye combinations, especially if the salt already holds water of crystallization.
Further, many of the deliquescent salts when they absorb moisture and melt can cause corrosion to the products stored in the shipping containers in which the humidity indicator cards are utilized.
In addition, the blotter paper used to form conventional humidity indicator cards sometimes sheds paper fibers and lint. Such fibers and lint may damage products that are sensitive to dust, such as electronic components.
Accordingly, it is an object of this invention to produce an irreversible humidity indicator card which solves the problems present with conventional irreversible humidity indicator cards.
It is a still further object of the invention to disclose an irreversible humidity indicator card which is formed of a composite structure which includes a darkened blotter paper which is useful in showing a predetermined level of humidity.
It is a further object of the invention to disclose an irreversible humidity indicator card which does not use a dye with its deliquescent material.
It is a further object of the invention to disclose an irreversible humidity indicator card which does not produce paper fibers or lint when in use.